body{
    margin: 0;
}

.box{
    width: 600px;
    height: 400px;
    background-color: #fc3;
    /* 绝对定位，不能采取margin居中 */
    margin: 100px auto;
    position: relative;
}

.box div{
    width: 200px;
    height: 100px;
    background-color: #f00;
    position: absolute;
    top:200px;
    left: 250px;
}